When she was a “munchkin,” Isla Fisher loved the tooth fairy, so she was really excited when she was asked to voice the character in DreamWorks’ new animated 3-D production of “The Rise of the Guardians,” which opens Wednesday.

The film is directed by Peter Ramsey and is based on the book series “The Guardians of Childhood” by William Joyce, as well as on the author’s short film “The Man in the Moon.”

Isla Fisher is the voice of the Tooth Fairy in “The Rise of the Guardians,” which opens in theaters on Wednesday, Nov. 21, 2012. (Mathieu Young)

In the script by David Lindsay-Abaire (“Robots,” “Rabbit Hole”), a motley collection of childhood heroes – Santa Claus (Alec Baldwin), the Easter Bunny (Hugh Jackman), the Tooth Fairy (Fisher) and the Sandman, who doesn’t talk – are drawn together by the moon when the bogeyman, Pitch (Jude Law), intends to conquer the world by spreading fear. The only thing that can counter his power is belief in the mythical superheroes by the children of the world.

To the surprise of the others, the moon wants the heroes to include Jack Frost (Chris Pine) – a free spirit very much like Peter Pan – who suffers from an emotional trauma he suffered when he was created.

“It’s an incredibly cool film,” says Pine about “Guardians,” admitting he was afraid of the bogeyman as a kid. “Guillermo del Toro is behind it, and the animation is very interesting. It’s nothing like anything anyone has ever seen.”

Del Toro was one of the executive producers of the film, which was produced by Christina Steinberg (“National Treasure,” “Bee Movie”), and Nancy Bernstein (“The Lord of the Rings: The Fellowship of the Ring”).

Ramsey, who came to the project three years ago, around the same time as del Toro, says the biggest thing the filmmaker did “was to inspire us to keep the movie cinematic, to not get lost in the weeds of what it is to do an animated movie. … He really kept the creative fire going for us.”

“Guardians” is the first feature for Ramsey, a longtime storyboard artist and assistant director who says he had to pinch himself every day because of the cast he had. But he notes a lot of work had been done on the characters beforehand and that the challenge was to find “actors who were already those characters in a way.”

With Pine, they had seen him in “Star Trek.”

Chris Pine is the voice of Jack Frost in “The Rise of the Guardians,” which opens in theaters on Wednesday, Nov. 21, 2012. (Mathieu Young)

“Naturally, everyone had kind of fallen in love with him there,” Ramsey says. “With Jack Frost he has this classic hero’s journey story. A lot of that was echoed in `Star Trek.’ But it wasn’t until we got an animation test with Chris’ voice coming out of Jack that the chill started going up our spines.”

The one thing they were concerned about was that Jack looks kind of waifish and elfin and Chris has a “husky thing going on with his voice.

“But somehow hearing him as Jack Frost makes it seem kind of timeless,” Ramsey says. “Chris is such a great actor, we started calling it method animation because of the way he would talk about the emotional content of the scene.”

As for Fisher, they knew she could be kooky and crazy from “Wedding Crashers.” There was even an animation test using her voice from “Wedding Crashers” that was “pretty hilarious,” Ramsey says.

“But what really blew me away about her was the emotional stuff,” he says. “There’s a few scenes in the movie where she’s heartbroken or commiserating with Jack Frost that just melt my heart. She’s got this quality to her voice that is childlike but also has an ageless kind of maturity to it that really hits the bull’s-eye.”

The Tooth Fairy also comes off as a bit of a flirt.

“As flirtatious as you can be without legs and half a bird body,” Fisher jokes. “She does try and seduce Jack Frost, which is bizarre given their intraspecies differences.”

As is usual in animation, Fisher didn’t work directly with any of the other actors in the film.

“I worked with a very short thing called a microphone and the director and that was it,” says the Australian actress. “It was a lonely gig, but at the same time at least I can pull lots of silly faces and not feel self-conscious.”

However, she didn’t get to use her Aussie accent.

“Hugh selfishly got to do that,” she says of Jackman. “I would’ve loved to have done the Australian accent.”

And though she didn’t want to stray from Joyce’s vision, Fisher says she was able to give the Tooth Fairy “a type A personality when she’s in work mode and a little bit of a ditzy sweet, flirtatious thing when she wasn’t.”

Though now known for her comedies, Fisher was doing dramas early in her career when her now husband, Sacha Baron Cohen, suggested she try comedy. She was immediately cast in “Wedding Crashers.”

“It’s my personality, I’ve always been comfortable tapping into my inner idiot,” she says with a laugh, adding that coming from a big family she used humor growing up to gain attention.

Nevertheless, her next two films are dramas, Baz Luhrmann’s highly anticipated “The Great Gatsby” with Leonardo DiCaprio and the heist film “Now You See Me” with Mark Ruffalo, Jesse Eisenberg and Morgan Freeman.

A mother of two, Fisher takes her time between projects.

“My kids are my priority,” she says. “Like anyone who has a family of young people knows, it’s way too much fun to hang out with them. There’s not a job in the world that can compare.”

Though she thinks her children – ages 2 and 5 – may be a bit young for “Rise of the Guardians,” she loves what it’s about.

“If you don’t believe in fear, it doesn’t exist,” she says. “I think we live in quite fear-mongering times, whether it’s economical or whatever. It’s tough, and I think it’s pretty good for kids to get that message.”

Ramsey’s children are older – 20 and 16 – but when they were young, he would dress up as Santa Claus because his wife is Swedish and her tradition is that the jolly old man would deliver gifts on Christmas Eve.

“They really did believe. The same thing with the tooth fairy and the Easter bunny,” says the director. “We did it all.”

Ramsey sees the “Rise of the Guardians” story as a big metaphor for the power of the imagination to create what is best in life.

“The guardians clearly represent things like hope, wonder, dreams, memory,” he says, “all the things you have to nurture and protect in childhood in order to fight against fear and to live a full life.”